Got this info from a group I belong to:
Time: September 27, 2008
Location: Various Cities
Here's a great deal for budget-minded culture lovers: on Saturday, September 27th, the Smithsonian Magazine has arranged for free admission to hundreds of museums and cultural sites around the country as part of its Museum Day promotion. You're sure to find one within a reasonable drive from your home.
Print pass herehttp://microsite.smithsonianmag.com/museumday/about.html
Find local museum herehttp://microsite.smithsonianmag.com/museumday/venue.html
(We're going to the Zoo!)